1. Field of the Invention
The present invention relates to a coupling structure for airframe components such as wing surface panels constituting the airframe of an aircraft.
2. Description of the Related Art
In general, a wing constituting the airframe of an aircraft has a hollow structure, and a surface of the wing is formed of a plurality of wing surface panels. Then, two adjacent wing surface panels are coupled to each other by a fastener member (securing piece).
At this time, the two wing surface panels are coupled to each other by the fastener member in the following manner. That is, a pin-shaped fastener main body is inserted from the outer side of the wing into through-holes that are respectively formed on both the wing surface panels placed on top of each other. Then, the leading end part of the fastener main body is fixed by a fixing piece on the inner side of the wing.
Meanwhile, aircraft need to be fully prepared for a lightning strike. If lightning should strike an aircraft and a large current flows into a main wing of the aircraft, a part of, or in some cases the entirety of, the large current flows into a coupling portion between wing surface panels thereof. If the current value exceeds the threshold limit of a tolerable amount of current flowing through each coupling portion, a discharge phenomenon called an electric arc (or thermal spark) occurs. In this phenomenon, the current flowing through each coupling portion rapidly and locally increases the temperature of a boundary surface between the wing surface panels constituting the coupling portion and mainly made of a conductive material, and melts the boundary surface, so that a discharge occurs in the surrounding atmosphere. If this phenomenon occurs, melted substances called hot particles are scattered from the melted portion in many cases.
In general, the internal space of the wing also functions as a fuel tank. Hence, it is necessary to suppress the electric arc from occurring or seal the electric arc, to thereby prevent the generated arc discharge and the hot particles scattered thereby from coming into contact with flammable fuel vapor, at the time of the lightning strike.
Accordingly, for the purpose of lightning protection (explosion protection), it is necessary to reliably suppress the electric arc from occurring or seal the electric arc at a portion that can be filled with the flammable fuel vapor on a boundary surface between the fastener member and the wing surface panels coupled by the fastener member. Examples of the portion that can be filled with the flammable fuel vapor include the inside of the fuel tank, the inside of a surge tank generally disposed on a wing end side of the fuel tank (a tank in which a vent scoop and a burst disk are disposed), and the inside of fuel-system equipment, or the like.
Under the circumstances, the electric arc is sealed conventionally in the following manner. That is, the leading end part of the fastener member is covered with a sealant on the inner side of the wing. Alternatively, the leading end part of the fastener member is provided with a cap, and the gap between the cap and the fastener member is filled with an insulating material (for example, air). (See, for example, Japanese Patent Laid-Open Nos. 02-7398 and 2010-254287.)
In addition, in order to enable the fastener member to pass through members to be fastened, a technique called an interference fit or transition fit is generally adopted. In this technique, the inner diameters of through-holes formed in the members to be fastened and the outer diameter of the fastener member are set to fit dimensions based on the interference fit or a tolerance equivalent thereto, whereby a gap is prevented from being formed between the fastener member and the members through which the fastener member passes.
In this regard, as illustrated in FIG. 5A, if a gap 4 exists between a fastener member 1 and through-holes 2a and 3a which are respectively formed in wing surface panels 2 and 3 and through which the fastener member 1 passes, a lightning current L flowing along a surface of the wing surface panel 3 at the time of a lightning strike flows from the fastener member 1 into the wing surface panel 2 on an internal space 5 side of the wing. Consequently, an electric arc A may occur at a boundary surface between the fastener member 1 and the wing surface panel 2 on the internal space 5 side of the wing. Note that an insulating layer 7 of a primer or any other materials is provided between the wing surface panel 2 and the wing surface panel 3 in a portion other than the through-holes 2a and 3a. 
In contrast, the interference fit or transition fit is adopted, and as illustrated in FIG. 5B, the fastener member 1 is brought into close contact with the through-holes 2a and 3a which are respectively formed in the wing surface panels 2 and 3 and through which the fastener member 1 passes. This allows the lightning current L flowing along the surface of the wing surface panel 3 at the time of the lightning strike to flow from the fastener member 1 into the wing surface panel 2, inside (in a middle portion in the thickness direction) of the wing surface panel 2 located on the internal space side of the wing. In this way, the occurrence of the electric arc A as illustrated in FIG. 5A is suppressed.
Meanwhile, in order to suppress the occurrence of the electric arc A by adopting the interference fit or transition fit, the precision of the inner diameters of the through-holes 2a and 3a formed in the wing surface panels 2 and 3 are extremely important to prevent the gap 4 from being formed at the time of attaching the fastener member 1.
Accordingly, when the through-holes 2a and 3a are respectively formed in the wing surface panels 2 and 3, if a processing error occurs or the inner diameter is made excessively large by an error in dimensions, the gap 4 is formed. The electric arc A occurring in a portion in which a large current flows has correspondingly large energy. Hence, in the case where an electrical conduction function of the fastener member 1 is impaired, the sealant or the cap provided at the leading end part of the fastener member may not reliably suppress the occurrence of the electric arc A.
In addition, in an actual case, it is preferable to use in combination a plurality of occurrence suppressing means and sealing means for an electric arc, in order to increase the reliability. Therefore, electrical conduction on the plane on which the wing surface panels 2 and 3 abut against each other may be achieved in some cases by bringing the wing surface panels 2 and 3 into surface contact (so-called fay bonding). If the electrical conduction is partially impaired by some kind of trouble in processing, however, the resistance of this portion with the impaired electrical conduction increases, and the sealant or the cap may not reliably suppress the occurrence of the electric arc A. In addition, a sealant 8 is applied to a boundary portion between an end part 2b of the wing surface panel 2 and the wing surface panel 3 so as to cover the boundary portion, so that a so-called a fillet seal is formed. In the case where the fillet seal is insufficient, the electric arc A may not be suppressed from occurring from the boundary portion between the end part 2b of the wing surface panel 2 and the wing surface panel 3, depending on the level of the flowing current.
Accordingly, in the case of using in combination the plurality of occurrence suppressing means and sealing means for an electric arc, even if any of the occurrence suppressing means and sealing means for an electric arc cannot provide a sufficient function for some reason, it is preferable to suppress the occurrence of the electric arc so as to compensate the insufficient function.
The present invention has been devised on the basis of such a technical problem as described above, and therefore has an object to provide a coupling structure for airframe components that is capable of ensuring sufficient lightning protection capability.